A number of image processing techniques for performing frequency emphasis to improve the radiographic image diagnostic performance have been proposed. Frequency emphasis or suppression is performed by generating a plurality of band-limited images representing frequency components in a certain limited frequency band based on an original image, and then emphasizing or suppressing frequency components for each band-limited image. Methods for generating a plurality of band-limited images utilize Laplacian pyramid decomposition, wavelet transform, and unsharp masking. For example, when unsharp masking is used, a band-limited image HLv is represented by formula (1).[Math. 1]HLv(x,y)=SusLv-1(x,y)−SusLv(x,y)  (1)where SusLv indicates a defocused image.
A defocused image Sus0 with Lv=0 indicates an original image Sorg. Lv is an index for band-limited images. Generating defocused images having different frequency response characteristics enables acquiring various band-limited images. When a band-limited image having the lowest frequency is referred to as a low-frequency image L, a relation between the low-frequency image L and the original image is represented by formula (2).
                    [                  Math          .                                          ⁢          2                ]                                                                                  S            org                    ⁡                      (                          x              ,              y                        )                          =                                            ∑                              Lv                =                1                            LvMax                        ⁢                                                  ⁢                                          H                Lv                            ⁡                              (                                  x                  ,                  y                                )                                              +                      L            ⁡                          (                              x                ,                y                            )                                                          (        2        )            
This means that summing up decomposed band-limited images reconstructs the original image. Band-limited images other than the low-frequency image L are referred to as high-frequency images. Applying gain adjustment to high-frequency images by using a gain coefficient α according to formula (3) enables generating images processed through various types of frequency emphasizes or suppressions.[Math. 3]H′Lv(x,y)=HLv(x,y)+(αLv−1)×HLv(x,y)  (3)
The gain coefficient α is set for each frequency band. Increasing the gain coefficient α enables emphasizing the relevant frequency band component. Decreasing the gain coefficient α enables suppressing the relevant frequency band component. However, emphasis processing based on the constant α causes identical emphasis processing for all components. Specifically, emphasis processing based on the constant α emphasizes not only edge components (target of emphasis) but also noise components, causing a problem that a desired effect of emphasis cannot be acquired. To cope with the problem, there has been discussed a technique for detecting only edge components from high-frequency images and emphasizing edges to acquire an effect of emphasizing only edges (refer to Japanese Patent Application Laid-Open No. 9-248291 and Japanese Patent Application Laid-Open No. 2005-296331). In particular, Japanese Patent Application Laid-Open No. 2005-296331 discusses a technique for decomposing an original image into a plurality of band-limited high-frequency images and performing emphasis processing, i.e., a technique for detecting edges and applying emphasis processing to edges for each frequency band.